An important practical aspect of modern energy storage devices is ever-increasing energy density and power density. Safety has been found to be a major concern. Lithium ion cells currently in wide-spread commercial use are among the highest energy density batteries in common use and require multiple levels of safety devices, including external fuses and temperature sensors, that shut down a cell in case of overheating before a short circuit can occur as a result of the mechanical failure of the battery separator. Lithium-ion (Li-ion) batteries are also subject to explosion and fire should a short circuit occur because of mechanical or thermal failure of the separator. Li-ion secondary batteries present special challenges concerning durability over many cycles of charge and discharge. Commercially available Li-ion batteries typically employ microporous polyolefin (for example polypropylene or polyethylene) as a battery separator. Microporous polyolefins begin to shrink at 90° C., limiting the battery fabrication process, the use temperature of the battery, and the power available from the battery.
This situation reveals two key problems with the current state of separators for advanced Li ion batteries. First, microporous membranes made out of polyolefins (polyethylene, polypropylene) provide good structural and barrier properties along with good shutdown capabilities, but they do not provide high temperature stability and low shrinkage and thus are limited on safety performance of higher energy density batteries. Porous structures such as nonwovens that are stable at high temperatures offer low shrinkage and good high temperature stability but they do not offer good shutdown capabilities. Therefore, there is a need for battery separators that combine good barrier properties, shutdown capabilities and high temperature stability in order to maintain the performance and improve the safety of advanced Li ion batteries.
Combinations of these types of materials may offer a balance of these properties. The problem to be solved here is to create a bonding process that will result in good adhesion between the nonwoven layer and the microporous membrane layer while also minimizing any reduction in permeability or increase in resistance.